In Raman spectroscopy, the molecular fingerprint region is in the range of 700 to 1800 cm−1. Many different vibrations, including C—O, C—C and C—N single bond stretches, C—H bending vibrations, and some bands due to benzene rings are found in this region. However, many functional groups exhibit vibrational peaks at similar Raman shifts regardless of the structure of the rest of the molecule. For example, C—H stretching vibrations usually appear between 2800 and 3200 cm−1 and carbonyl (C═O) stretching vibrations usually appear between 1600 and 1800 cm−1. Raman spectra of hydrocarbon molecules tend to be dominated by strong features in the C—H stretch band. However, weaker peaks in the fingerprint region and elsewhere can be better suited to identification and quantitation of multiple components of a hydrocarbon sample stream. Practical examples of industrial applications for Raman analyzers involving hydrocarbon quantitation include natural gas transfer and synthetic fuels manufacturing.
In many hydrocarbon applications of Raman spectroscopy, the sample stream consists of many different components such as those listed in the table of FIG. 1. The different components may be present in widely varying relative concentrations, from very strong to very weak. The purpose of the analyzer is normally to measure each of those relative concentrations with as high accuracy as possible. This enables the analyzer to assist with the control of process parameters in synthetic fuels manufacturing, or to measure the energy content in a stream of natural gas. In the natural gas example, the dominant component by far is methane. Methane's primary peak for quantitation is in the C—H stretch region, where it can be an order of magnitude stronger than the next strongest features of the natural gas spectrum.
Further, many practical implementations of industrial Raman analyzers are capable of probing multiple sample streams simultaneously. This is typically done by feeding the Raman signals from each stream through individual fibers all coupled to a common spectrograph input. The spectrograph then disperses the spectrum of each sample stream onto different regions of a common CCD or other array detector camera. By the very nature of those array cameras, every detector on the array is read out with a common integration time. That integration time must be selected to be a) short enough so that the strongest signals of interest do not saturate the charge capacity of the detector or readout register, yet b) long enough so that the weakest signals of interest will rise above the electronic noise of the readout amplifier and the quantization noise of the analog-to-digital converter.
In such a multiple-sample analyzer application, the range of signal peak levels present within a given spectrum is further exacerbated by the fact that the separate spectra may be of very different overall magnitudes with respect to each other. This may be due to, for example, very different stream pressures, temperatures or compositions at the different sample points. As such, it is desirable to provide means for individually pre-conditioning each sample point spectrum upstream of the spectrograph such that a) very strong signals within a given sample spectrum, such as those within the C—H stretch region, are preferentially attenuated relative to the weak signals within that spectrum, and/or b) very strong sample point spectra are preferentially attenuated in a spectrally neutral sense with respect to the weaker sample point spectra in a multi-channel analyzer. Such means bring all spectral features of interest across all channels into better balance with the dynamic range of the array camera. This allows the inherently weaker signals to be measured with sufficient signal-to-noise ratio (SNR), while the inherently stronger signals (e.g. those in the stretch region and/or those from higher pressure sample streams) may simultaneously be measured to high SNR without saturating the camera.
FIG. 2 is an overlay of pure gas signals representing the component gasses of interest in a typical hydrocarbon application, obtained from an Optograf gas spectrum analyzer (Kaiser Analytics, Ann Arbor, Mich. 48103). As can be seen, the multi-component peaks in the C—H stretch region are significantly stronger and closer together than those of the fingerprint region. FIG. 3 is a detailed graph of the C—H stretch region.